1. Technical Field
The present invention is directed toward asphalt pavements, and more particularly toward an asphalt rejuvenater and an asphalt pavement composition including recycled asphalt pavement
2. Background Art
The following definitions will apply to terms used herein. Hot mix asphalt concrete ("HMAC") means an asphalt pavement for application to a roadbed. HMAC consists of three primary constituents: 1) aggregate of various geologic sources and sizes; 2) asphalt cement (or asphalt binder) which consists of asphaltine hydrocarbons; and 3) one or more modifiers to enhance water resistance or improve wear, and which could include shale oil modifier ("SOM") discussed in greater detail below. Recycled asphalt pavement (RAP) means HMAC which has been milled off an existing paved roadbed. HMAC containing RAP means hot mix asphalt concrete which includes recycled asphalt pavement During the past several years, the recycling of asphalt pavements has become a more commonly practiced and attractive pavement rehabilitation technique. In 1993, the U.S. Government reported that 91 million tons/year of recycled asphalt pavement ("RAP") were being produced in the U.S., of which 73 million tons/year were being recycled for highway applications.
There are several obvious benefits associated with pavement recycling. Typically, and from a cost point of view, the use of RAP results in a lower first cost of rehabilitation due to the savings realized from the reutilization of materials (i.e., aggregate and asphalt binder). From a structural engineering point of view, this technique permits the removal of all or a portion of the defects in the preexisting pavement, which defects might otherwise reduce the effectiveness and longevity of resurfacing treatments. Moreover, this technique permits the preservation of the surface geometry, which often is important in cases where changes in elevation cannot be tolerated or are too expensive to accommodate. Lastly, the reuse of the materials removed from the preexisting pavement offers certain environmental and conservation benefits through a reduction in the problems associated with the disposal of this material and through the utilization of the asphalt cement or asphalt binder content of the RAP.
The techniques presently used for the recycling of asphalt pavements commonly are classified as surface recycling, in-place surface and base recycling and central plant recycling. Surface recycling involves the application of a rejuvenating treatment to the surface of the existing pavement. In-place recycling refers to the removal and immediate replacement of some or all of the preexisting pavement. Central plant recycling, as the name implies, refers to the removal of some or all of the existing pavement to a central plant location, where all or a portion of this material is reprocessed with varying proportions of virgin aggregate and/or virgin binder and then reused for paving applications. Each of these techniques may employ either a hot or cold process.
The use of RAP presents several technical challenges. In this regard, the most likely reason that an asphalt pavement is in need of rehabilitation is due to the aged, or hardened, condition of its asphalt binder, which is manifested in the degradation of the rheological properties of the asphalt binder, in particular, an increase in viscosity. This condition reduces the pavements ability to resist the loss of adhesion between the binder and the aggregate and to resist cracking due to fatigue and thermal cycling. Hence, and in order for the preexisting pavement to be effective for reuse, some process must be devised to restore the Theological properties of the aged binder in the RAP to a condition that is more typical of an unaged, or virgin, binder. In the industry, the term of art that commonly is used to describe this process of restoration is "rejuvenation".
While several methods of rejuvenation have been devised and practiced, generally all involve the application of heat or the use of a rejuvenating agent. With regard to the latter, there are a myriad of products presently being marketed and used in the U.S., which products generally are classified as either flux oils, viscosity graded asphalt cements, emulsified asphalts or a host of brand-named products of proprietary formulation. The selection of the type and usage rate of these products, for the most part, is based on the properties of the aged asphalt in the RAP, the percentage of RAP to be used in the new mix, and the recycling technique to be employed.
There are certain problems inherent in the in-place methods. Absent the addition of significant quantities of virgin aggregate or virgin mix, the quantity of rejuvenating agent that can be added to the RAP without suffering a reduction in the stability and associated strength of the relaid HMAC containing RAP pavement is relatively small (i.e., typically less than 1% of the weight of the RAP). Stated differently, the asphalt content and aggregate gradation of the RAP often are such that very little, if any, rejuvenater can be added to the RAP without adversely affecting the void content, and thus the stability and strength, of the recycled mix Hence, it is not always possible to achieve the desired degree of rejuvenation when in-place methods of recycling are employed.
Furthermore, the degree of "apparent" rejuvenation that can be achieved with emulsified asphalts and many of the brand-named products is greater than the degree of rejuvenation demonstrated with low viscosity graded asphalt cements. The word "apparent" is used to reflect that the effectiveness of some of these products is reported to be temporary in nature, with little if any lasting effect on the restoration of the rheological properties of the aged asphalt. Nonetheless, in-place recycling methods typically employ some form of these products in an effort to effect the desired degree of rejuvenation.
When emulsified asphalts are used, care must be taken to ensure that the new mix has had sufficient time to "cure" before being subjected to traffic loading. While this problem can be mitigated through use of "rapid cure" emulsions, it is not uncommon for these recycled pavements to require several days, if not months, of curing before their maximum strengths are realized. In addition, caution must be exercised to ensure that the rejuvenating agent does not contain a high percentage of low boiling range volatiles, particularly when hot recycle processes are to be employed.
Because of these problems with in-place recycling techniques, central plant recycling techniques offer certain advantages, particularly with regard to mix design and quality control. Although the initial cost of central plant recycling may exceed that of in-place recycling techniques, if the problems associated with in-place recycling result in a pavement with reduced service life, then its advantage of lower first cost may be negated by the higher cost of future rehabilitation requirements.
One problem with central plant recycling techniques is that the addition of even low viscosity graded asphalt cements ("AC's") often fails to meet the objective of rejuvenating the Theological properties of the aged asphalt in the RAP to close to that of virgin asphalt. As a result, the viscosity of the binder in the recycled asphalt pavement is higher than desirable, leading to reduced resistance to thermal and fatigue cracking and reduced service life.